This invention relates to tone signal detectors for paging receivers, transceivers, mobile radio equipment and so forth, and more particularly it pertains to digital tone signal detectors having N-path filters.
A conventional tone signal detector (reference is made to U.S. Pat. No. 3,882,466, 3,803,429 or 3,670,242) is composed of selective elements (for example, active filters) for selecting tone signals, an amplifier for amplifying these output signals and an analogue detecting circuit for converting these output signals into D.C. voltages and giving detecting pulses. Since the recent increase in number of channels (radio frequencies) per receiver has resulted in a greater number of selective elements required and the size of receivers has also enlarged, reductions are called for in the size of selective elements and detecting circuits and in the number of selective elements. To satisfy these needs, some tone signal detectors are provided with a variable-frequency active filter in place of a plurality of selective elements, and the pass frequency of the active filter is switched from one to another of the tone signal frequencies by an electronic circuit. However, since the center frequency, band width and voltage gain of such an active filter are dependent on the performance of resistors and capacitors constituting it, the resistances and capacitances, respectively, of these resistors and capacitors have to be highly precise, a requirement which hampers the use of integrated circuits, mass-production and cost reduction.
Moreover, even where an active filter is used, the filter output is often handled in practice by an analogue detecting circuit.